The present invention relates to a workpiece spindle control unit and a control method suitable for application to a workpiece spindle capable of performing C-axis control machining with a workpiece held thereby.
It is known these days to be able to index a workpiece spindle at a high speed and high accuracy in C-axis control, in such a manner that a workpiece motor, and a workpiece spindle is directly rotated and driven by a workpiece spindle motor in a complex machining machine tool capable of performing turning and C-axis control machining.
In such a machine tool, a method of performing machining by fixing a workpiece spindle at a predetermined C-axis angular position, according to C-axis control, is frequently used in the case of milling and the like. In this case, a workpiece spindle is fixed by using a clamping means comprising a brake installed on the workpiece spindle. The workpiece spindle may be shifted from an angular position originally positioned by an unbalanced contact of brake friction pads during clamping, owing to the assembly accuracy of the clamping unit. This results in deterioration of the machining accuracy. In the clamping with a clamping unit, when a spindle driving motor, that is, a workpiece spindle, is clamped driving a position loop in servo-control, excessive current flows in the motor in order to absorb the servo positioning error, and the motor overheats. This method is open to objections. It is thinkable to control by cutting the servo during clamping in order to prevent overheating. When a servo is cut, it is impossible to return to a correct C-axis position without performing an original reversion movement during unclamping. Moreover, such a method is also used that a rotation only signal is detected from a rotation detector side after the clamping of a motor, and the control according to the rotation signal isn't performed during clamping, but after unclamping of a motor. This control method needs complex control procedures such as switching of a servo between ON and OFF, and the chasing of a rotation signal. Therefore, it is inconvenient.
An object of the present invention is to provide a workpiece spindle control unit and its control method capable of clamping at a correct angular position.
A further object of the present invention is to provide a workpiece spindle control unit and its control method for which an origin reversion movement is unnecessary in unclamping of a spindle driving motor, and complex control procedures, such as the switching of a servo between ON and OFF, are also needless.